The present invention relates generally to silencers for air-moving devices and specifically to a method and apparatus to reduce fan noise of a thermal management system using resonators integrated with fan shrouds and barrels.
In an effort to find new energy sources, fuel cells using an electrochemical reaction to generate electricity are becoming an attractive energy alternative. Fuel cells offer low emissions, high fuel energy conversion efficiencies, and low noise and vibrations. U.S. Pat. No. 5,248,566 to Kumar et al. These advantages make fuel cells useful in automotive applications. Of the various types of fuel cell types, the proton electrolyte membrane (PEM) fuel cell appears to be the most suitable for use in automobiles, as it can produce potentially high energy, but has low weight and volume.
One design challenge for a vehicle with a PEM fuel cell stack is the high amount of heat it produces while in operation. Thermal management systems (coolant systems) are known both for conventional vehicles and even for fuel cell vehicles. A fan is usually situated behind a heat exchanger such as a radiator to draw a large quantity of air through the radiator to cool a coolant that travels through a closed loop from the fuel cell stack. Similar configurations exist for coolant systems of internal combustion engines.
Unfortunately, noise levels associated with powerful fuel cell coolant system fans are often much higher than acceptable to most operators. Successful implementation of a fuel cell vehicle will require a system and method to significantly reduce this fan noise. Reduced noise would also benefit any coolant system using a fan or fans.
Devices are known in the prior art to reduce fan noise in vehicle coolant systems. U.S. Pat. No. 6,082,969 to Carroll et al. describes forwardly skewed fan blades of an axial flow fan behind a radiator with an increasing blade angle to reduce noise levels. Enclosures using ducts or baffles can also reduce sound/noise but are generally impractical for vehicle applications due to their large size especially if designed to reduce low frequency noise levels. See generally, U.S. Pat. No. 5,625,172 to Blichmann et al.
Noise reduction using a tuned Helmholtz resonator is also known in the art. The resonator has an air space (volume) that communicates with the “outer air” through an opening. An air plug present in the opening forms a mass that resonates on support of the spring force formed by the air enclosed in the hollow space/cavity. The resonant frequency of the Helmholtz resonator depends on the area of the opening, on the volume of the air space, and on the effective length of the air plug formed in the opening. When either the volume of the air space or the effective length of the air plug becomes larger, the resonant frequency is shifted toward lower frequencies. When the area of the opening is made smaller, the resonant frequency is shifted towards lower frequencies.
When Helmholtz resonators are driven with acoustic energy at a resonant frequency, the resonators will absorb a maximum amount of the incoming acoustic energy. Nevertheless, because they are tuned systems, the absorption decreases as the frequency of the incoming acoustic energy varies from the predetermined resonant frequency. Thus, the principle limitation with these devices is their ability to attenuate sound energy efficiently only within a limited frequency range. Therefore, to work effectively, a plurality of differently tuned Helmholtz resonators would be needed for broadband noise applications.
The capability of Helmholtz resonators to attenuate noise in long pipes had been demonstrated in internal combustion engine air intake and exhaust systems. It is unknown in the art to use Helmholtz resonators in a shroud around an air-moving device such as a fan placed near a radiator of a vehicle coolant system. This would provide an effective and low cost means to reduce fan noise associated with these applications.